Hot water from the house supply isn't that hot though?

Depends on how you have it set. My current and previous hot water heaters had thermostats which permitted adjusting the temperature.

They also had warning labels on them about scalding water. If it's hot enough to scald, it's hot enough.

The dishwasher also doesn't need to heat up a "bulk" amount of water, just the amount of water used for washing the current load of dishes.

If you're washing dishes and someone is, or has recently, taken a shower; or someone is, or has recently, done laundry; or someone is, or has recently shaved or done any of the other dozen things that draw from the hot water heater, then the water is already hot and available and doesn't need to be heated all the way from cold by the dishwasher. A properly insulated hot water heater can retain heat for quite some time.